1. Field
The present disclosure relates generally to electronics, and more specifically to transmitters and receivers.
2. Background
In a radio frequency (RF) transceiver, a communication signal is typically received and downconverted by receive circuitry, sometimes referred to as a receive chain. A receive chain typically includes a receive filter, a low noise amplifier (LNA), a mixer, a local oscillator (LO), a voltage controlled oscillator (VCO), a baseband filter, and other components, to recover the information contained in the communication signal. The transceiver also includes circuitry that enables the transmission of a communication signal to a receiver in another transceiver. The transceiver may be able to operate over multiple frequency ranges, typically referred to a frequency bands. Moreover, a single transceiver may be configured to operate using multiple carrier signals that may occur in the same frequency band, but that may not overlap in actual frequency, an arrangement referred to as non-contiguous carriers.
In some instances, it is desirable to have a single transmitter or receiver that is configured to operate using multiple transmit frequencies and/or multiple receive frequencies. For a receiver to be able to simultaneously receive two or more receive signals, the concurrent operation of two or more receive paths is required. Such systems are sometimes referred to as “carrier-aggregation” systems. The term “carrier-aggregation” may refer to systems that include inter-band carrier aggregation and intra-band carrier aggregation. Intra-band carrier aggregation refers to the processing of two separate and non-contiguous carrier signals that occur in the same communication band. Currently, even though these non-contiguous carriers may be close together, a separate receive chain is typically needed to process each carrier. When implementing such a carrier aggregation receiver, it is possible that power from one carrier may interfere with the down-conversion of another carrier, causing what is referred to as intermodulation distortion. Intermodulation distortion occurring as a second-order function is referred to as IM2 and when occurring as a third-order function is referred to as IM3. Intermodulation distortion can lead to desensitizing the receiver, a condition sometimes referred to as “receiver desensitization” or “receiver desense.” Receiver desensitization may occur due to the presence of second-order distortion, third-order distortion, and higher-order distortion caused by the operation of the low noise amplifier (LNA) or may occur as a result of an out of band (OOB) jammer signal, such as a WIFI jammer signal or an OOB transmit jammer signal.
WIFI is a term given to a relatively short-range local area network generally complying with IEEE 802.11 protocols, operating in a number of different frequency bands. Modern cellular transceivers need to operate in the presence of WIFI transceivers. Cellular receiver desensitization may occur due to the presence of out-of-band jammers in some or all of the WIFI bands. A WIFI transceiver may operate in a frequency range that may overlap with one or more cellular transceiver frequencies. For example, a WIFI transceiver's frequency range may have transmit energy in the 2.4 GHz range and in the 5.25-5.725 GHz range, which may overlap with the 3rd order local oscillator (LO) range of a cellular transceiver operating in low band (LB) B20 (having a downlink frequency between 791 MHz-821 MHz) and a cellular transceiver operating in mid band (MB) B2 (having a downlink frequency between 1930 MHz-1990 MHz) and B3 (having a downlink frequency between 1805 MHz-1880 MHz). In this example, the LO of the cellular receiver will downconvert the desired fundamental signal, along with the WIFI OOB jammer occurring at the 3LO frequency, leading to receiver sensitivity degredation.
Furthermore, if a WIFI jammer occurs in a receiver configured for carrier-aggregation, the WIFI jammer power may be inter-modulated with transmit signal leakage into a receive band, with the result of 2nd or 3rd order intermodulation products appearing in a receive band.
At low power levels, the fundamental output power of the LNA rises in a one-to-one ratio (in terms of dB) with respect to the input power, the second-order output power rises in a two-to-one ratio, and the third-order output power rises in a three-to-one ratio. If the input power is high enough for the LNA to reach saturation, the output power flattens out in all the first-order, second-order and third-order cases.
The second order intercept point (IIP2) is the output power point at which the extrapolated first- and second-order lines intersect on a plot, since the actual power levels will flatten off due to saturation at a much lower power level typically. The third-order intercept point (IIP3) is the output power point at which the extrapolated first-order and third-order lines intersect on a plot, since the actual power levels will flatten off due to saturation at much lower power level typically.
Further, LNA out-of-band (OOB) IIP2 and IIP3 performance becomes more of a concern when the LNA input matching circuitry moves toward single-element, such as single inductance (L) components, particularly when the single-L matching circuitry is located on a different chip than the LNA. When implementing such a single-L matching circuit, higher order harmonic cancelation becomes more difficult.
Thus, it would be desirable to be able to cancel higher order harmonics and improve receiver out-of-band linearity (IIP2 and IIP3) simultaneously when implementing a single-L matching circuit for a receiver.